Air pollution control

ABSTRACT

In a method of controlling air pollution, a combustion gas is passed through a slurry of an alkaline-earth metal sulfide and a redox buffer (e.g., a phosphate), preferably in combination with an alkaline-earth metal carbonate and/or hydroxide. A composition for removing a heavy metal from a combustion gas is also provided, as an aerosolized, aqueous dispersion or slurry of an alkaline-earth metal sulfide and a buffering agent. A solid residue of the air pollution control process is also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority of U.S. ProvisionalApplication Nos. 60/566,272 (filed Apr. 29, 2004) and 60/646,263 (filedJan. 24, 2005). The disclosures of both applications are incorporated byreference as if set forth herein in their entirety.

FIELD OF THE INVENTION

The present invention is directed generally to air pollution controltechnology, and in particular to technology for reducing mercury andother pollutants from combustion gases.

BACKGROUND OF THE INVENTION

Combustion gases from incinerators, power plants, and coal-firedfurnaces typically contain oxides of sulfur (SO_(x)), oxides of nitrogen(NO_(x)), and volatile heavy metals such as mercury. Typical mercuryconcentrations in coal are 0.05 to 0.25 mg/Kg. On combustion, themercury is volatilized and carried in the combustion exhaust gases.

Mercury poses a serious problem for human beings and the environment.MSNBC.com recently reported that the Southeast United States alone couldsave up to $2 billion a year by reducing mercury pollution. Coal-burningelectric power plants are the single biggest source of mercuryemissions, accounting for 40 percent of the total. Coal-fired burnersaccount for another 10 percent.

Prior efforts to control SO_(x), NO_(x), and mercury emissions haveincluded the use of dispersions of calcium carbonate and active carbonas an injected spray administered to exhaust gases, and/or the passingof exhaust gases through a scrubbing tower to neutralize the SO_(x) andNO_(x) emissions and sorb the volatile mercury. Following the scrubbingprocedure, solids carried in the exhaust can then be recovered byelectrostatic precipitators prior to discharge of the gases to theatmosphere. A combination of scrubbing and electrostatic precipitatorsused to condense gas-bound dusts can typically remove 50-85% of thegas-borne mercury.

Other efforts to control mercury emissions have employed alkali metalsulfides, e.g. sodium polysulfide solution, and in particular sodiumtetrasulfide solution. See, e.g., U.S. Pat. No. 6,214,304, and BabcockPower Environmental Inc. Technical Publication, “Multi-PollutantEmissions Control & Strategies, Coal-Fired Power Plant Mercury Controlby Injecting Sodium Tetrasulfide”; Licata A, Beittel R, Ake T, ICACForum, Nashville, Tenn. Oct. 14-15, 2003. Mercury is converted tomercury sulfide, and the mercury sulfide is precipitated out via a dustseparator. The process requires the alkali metal sulfide solution to beintroduced into the flue gas accompanied by the simultaneous addition ofheat. The process can be used in conjunction with the addition ofoxidizing agents, e.g. chlorine-containing compounds, to the burningcoal or coke, such that elemental mercury is converted into its oxidizedform, enabling the reagent to react with it more readily. This processis disadvantageous in that it is temperature-critical and the use ofoxidizing agents in coal feedstocks may increase corrosion within thesystem. Additionally, the process residues containing precipitatedmercury sulfide, in its black or beta-crystalline form, can be readilyoxidized, as mercury sulfide is not stabilized to pH- or redox-inducedreactions, thereby rendering the mercury sulfide precipitate vulnerableto re-oxidation and resolubilization in water, with the potential forincreased bioavailability.

SUMMARY OF THE INVENTION

The present invention addresses the problem of mercury—and indeed, otherheavy metals—in combustion gases by providing an improved process forremoving such pollutants, using readily obtainable reagents, techniques,and apparatuses, and which can be used with or in conventionalcombustion gas scrubbers, for example, flue gas desulfurization (FGD)scrubbers. According to a first aspect of the invention, a combustiongas is allowed to contact a mixture of an alkaline-earth metal sulfideand a buffering agent, preferably a one selected from the groupconsisting of phosphoric acid, salts of phosphoric acid, alkaline-earthmetal-based pH buffers, and mixtures thereof. Advantageously, in oneembodiment of the invention the buffering agent comprises both analkaline-earth metal pH buffer (e.g., calcium carbonate) and a redoxbuffer (e.g., triple super phosphate) capable of stabilizing a mercurysulfide precipitate. The mixture of reagents can be formed as an aqueousdispersion or slurry and introduced as an aerosol into the combustiongas stream, preferably downstream of a particulate removal device, suchas a fabric filter or electrostatic precipitator (ESP).

In a second aspect of the invention, a composition for removing mercuryor other heavy metal(s) from a combustion gas is provided and comprisesan aerosolized, aqueous dispersion or slurry of an alkaline-earth metalsulfide and a buffering agent, for example, an aerosolized, 20-50% w/wsolids dispersion of an alkaline-earth metal sulfide and a bufferingagent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become better understood when reference is made tothe accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a wet scrubber for removingmercury from a combustion gas according to one embodiment of the presentinvention; and

FIG. 2 is schematic illustration of a dry scrubber for removing mercuryfrom a combustion gas according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

According to a first aspect of the invention, a method of controllingair pollution comprises allowing a combustion gas to contact a mixtureof an alkaline-earth metal sulfide and a buffering agent. Preferably,the buffering agent is selected from the group consisting of phosphoricacid, salts of phosphoric acid, alkaline-earth metal-based pH buffers,and mixtures thereof. The alkaline-earth metal sulfide and a bufferingagent (sometimes referred to collectively as a remediation agent) can beintroduced as an aerosol into a combustion gas to react with andfacilitate the removal of mercury and/or other heavy metals.

Nonlimiting examples of alkaline-earth metal sulfides include calciumsulfide, magnesium sulfides, adducts of calcium sulfide, adducts ofmagnesium sulfide, mixed calcium-magnesium sulfides, and mixturesthereof.

Nonlimiting examples of alkaline-earth metal-based pH buffers includecalcium carbonate, calcium hydroxide, calcium phosphate, magnesiumcarbonate, magnesium hydroxide, magnesium phosphate, mixedcalcium-magnesium carbonates, mixed calcium-magnesium hydroxides, mixedcalcium-magnesium phosphates, triple superphosphate, apatite, andmixtures thereof. Triple superphosphate (also known astrisuperphosphate, TSP, and superphosphate) is predominately monocalciumphosphate hydrate (CaH₂PO₄)₂.H₂O) (CAS No. 65996-95-4)).

In a preferred embodiment, the buffering agent includes at least oneredox buffer capable of stabilizing a mercury sulfide precipitateagainst subsequent redox reactions and/or dissolution in water(resolubilization). Nonlimiting examples of such redox buffers includephosphoric acid, salts of phosphoric acid, and mixtures thereof. Morepreferably, the buffering agent comprises at least one alkaline-earthmetal-based compound and at least one redox buffer.

Alkaline-earth carbonates, hydroxides, phosphates, and like materialsact as pH buffers within the acid gas stream, giving stability to thereagent mixture.

Phosphoric acid and/or its salts (e.g., phosphates) is employed as aredox buffer to ensure the stability of the final precipitated mercurysulfides. Additionally the phosphate may act as a moderate acid tooxidize any elemental mercury present in the combustion gas, such thatit can be more easily precipitated by the alkaline-earth metalsulfide(s). Phosphates also have the benefit of being corrosioninhibitors. In contrast, calcium chloride agents currently employed ascoal feedstock additives to facilitate elemental mercury oxidation cangive rise to increased risk of plant corrosion.

Phosphates may function both as pH buffers and redox buffers.

A number of generic and more specific examples of the mixtures ofreagents used in the practice of the invention are provided. In ageneric embodiment, the mixture of reagents comprises an alkaline-earthmetal sulfide and a buffering agent. In a slightly less genericembodiment, the mixture of reagents comprises an alkaline-earth metalsulfide, an alkaline-earth metal pH buffer, and a redox buffer. In aless generic embodiment, the mixture of reagents comprises analkaline-earth metal sulfide, an alkaline-earth metal carbonate, and aphosphate. In a more specific embodiment, the buffering agent comprisestriple superphosphate and at least one alkaline-earth metal carbonate orhydroxide (e.g., calcium carbonate, calcium hydroxide, and so forth). Inanother embodiment, the mixture of reagents comprises a mixture ofcalcium sulfide, triple superphosphate, and calcium carbonate. Even morespecifically, in one embodiment of the invention, the mixture ofreagents comprises calcium sulfide, triple superphosphate, and calciumcarbonate, present in relative amounts of 3:1:2 by weight (w/w),respectively. Such a mixture is available from Solucorp Industries, Ltd.(West Nyack, N.Y.), as MBS 2.1™.

The mixture of reagents can be provided as an aqueous dispersion orslurry of finely divided particles in water, and can be administered(introduced to a combustion gas) as an aerosol or spray, or in someother convenient manner. Advantageously, the dispersion or slurry can,and preferably does, include a small amount (parts-per-thousand) of adispersant, preferably a hyperdispersant, more preferably a polymerichyperdispersant, such as the “Solsperse” adducts from Noveon SpecialtyAdditives, Noveon Division, Lubrizol Ltd., Blackley, Manchester M9 8ZS,United Kingdom. A specific example is “SolPlus D540.” A solidsdispersion can be prepared by bead milling.

The dispersion or slurry of reagents is provided at a concentrationsuitable for use in a particular air pollution control apparatus, ortype of apparatus, such as a wet or dry scrubber. For example, where awet FGD-type scrubber is to be utilized, the mixture of analkaline-earth metal sulfide and a buffering agent can comprise a 20-50%w/w solids dispersion, and the dispersion can be injected from nozzlesinto a combustion gas stream. In a dry scrubber, lower concentrationdispersions or slurries may be appropriate, for example, a 20% w/wslurry of alkaline-earth metal sulfide and buffering agent, applied viaa rotary atomizer. For a dry scrubbing system, typical particle size(solids) is roughly <325 mesh (e.g., about 45 microns). For wetscrubbing systems, larger particles (<200 mesh, e.g., about 75 microns)are acceptable.

FIG. 1 illustrates one embodiment of a wet scrubber 10 suitable for usein a FGD process and in the practice of the present invention. A mixtureof reagents comprising an alkaline-earth metal sulfide and a bufferingagent resides in the additive feed tank 12, with or without additionalground lime and/or limestone. It is introduced into the sump 14 of thewet scrubber via valve C. The spray in the tower 16 is controlled viavalve A, and is regulated to meet the up-flowing combustion gases,downstream of a particulate removal device, such as a fabric filter orelectrostatic precipitator (ESP). The pH of the reaction tank 18 ismaintained at approximately pH 6 to 8. The reaction is monitored forsulfide content, and when this falls below effective levels the reactiontank is drained to the effluent transfer tank 20 via valve B, and theslurry is de-watered (thickened). Drained water is recycled in theprocess. The concentrated slurry is transferred to a press-filter orrotary drum filter (not shown) via valve H, and the dried solid ispackaged for sale or disposal. The reaction tank is recharged and theprocess recommences. A single scrubber can be operated in isolation or,more preferably, banks of three to six wet scrubbers are operated inparallel. They may be used in conjunction with existing gypsumproduction units or operated as a pre-treatment, upstream of a gypsumproduction unit or units.

Scrubbers for air pollution control are well known. A nonlimitingexample of one such scrubber is the wet flue gas desulfurization (FGD)scrubber sold by Babcock and Wilcox (a subsidiary of McDermottInternational). The scrubber has a spray level for introducing reagents,and includes at least one nozzle, i.e., a slurry spray nozzle.

FIG. 2 illustrates one embodiment of a dry scrubber 50 suitable for usein the practice of the invention. A mixture of reagents is introducedinto the dry scrubber as a 20% w/w slurry via a rotary atomizer 52. Theconfiguration for the scrubber 54 should be such as to maximize the wetcontact time of the reagent aerosol with the combustion gases, yetpermit the dried aerosol to pass into the bag house 56 in a dry form soas not to blind the bag house filtration system. The dried reactionproduct may be recovered for recycling or sale.

In both the wet scrubbing and dry scrubbing systems, mercury present inthe combustion gas is converted to a solid mercury sulfide precipitateand thereby separated from the exhaust gas. While not bound by theory,it is believed that the present invention should facilitate greater than95% recovery of mercury as the combustion gas transits the scrubber.Advantageously, the mercury-containing process residues are solid,substantially insoluble and, it is believed, stabilized againstresolubilization, as the buffering agents act to retard redox reactionsthat could lead to oxidation and resolubilization.

It will be appreciated that, in addition to mercury, other heavy metalspresent in a combustion gas should be recoverable through the practiceof the present invention. Most heavy metals form metal sulfideprecipitates. These should be stabilized against subsequent oxidationand resolubilization as a consequence of exposure to phosphates orsimilar buffering agents present in the remediation agent, allowing leadand other heavy metals to be recovered in a water-stable form.

It will also be appreciated that the removal of mercury and other metalsfrom a combustion gas according to the invention is compatible withexisting air pollution control equipment and methods for removing SO_(x)and NO_(x) gases from combustion gas.

In addition to the air pollution control processes described above, theinvention also provides a remediation agent for use with wet and dryscrubbers. According to this aspect of the invention, a composition forremoving mercury or other heavy metal(s) from a combustion gas comprisesan aerosolized, aqueous dispersion or slurry of an alkaline-earth metalsulfide and a buffering agent, where each of those terms is describedabove. In one embodiment, the aerosolized dispersion or slurry comprisesa 20-50% w/w solids dispersion of alkaline-earth metal sulfide and abuffering agent. Aerosols are formed in a conventional manner using,e.g., a spray nozzle, rotary atomizer, or other suitable apparatus.

The air pollution control methods and compositions described herein havethe advantages of not being temperature-critical and readilyfacilitating reaction with mercury and, indeed, other heavy metals inboth their ionic and elemental forms, at room temperature. In contrast,basic agents that act by virtue of their alkalinity alone cannot reactwith and remove elemental mercury from vapor. Processes employing sodiumsulfide are temperature-critical; their effectiveness requires theapplication of high temperatures.

The use of an alkaline-earth metal sulfide is significant. Essentiallyinsoluble and solid in form, alkaline-earth metal sulfides are much lessmalodorous than alkali sulfides, thereby rendering them easier tohandle, and they are less susceptible to the degradative oxidation thatis experienced by alkali sulfides such as sodium polysulfide and sodiumtetrasulfide.

The presence of alkaline-earth metal carbonates, hydroxides, and/orsimilar compounds within the remediation agent buffers thealkaline-earth metal sulfide from adverse oxidative interactions withacid compounds within the flue gas. Thus any unreacted alkaline-earthmetal sulfide, which is not diluted by degradative oxidation residues,can be recycled, and the evolution of hydrogen sulfide gas is minimized.

Additionally the process residues are solid, insoluble and stabilized.(Cf. Molecular Bonding Systems SITE Report EPA/540/R-97/507,incorporated by reference herein.) They are maintained in this form withthe assistance of a redox buffer, e.g. trisuperphosphate or calciumphosphate. Thus, an additional advantage and aspect of the presentinvention is that the by-products recovered from the scrubber andprecipitators are in a stable and non-leachable form, such that they maybe disposed of as non-hazardous waste or, alternatively, supplied as anon-hazardous raw material for the manufacture of construction products,such as gypsum board, cement, and cement blocks. Hence, the inventionalso provides a solid residue produced as a byproduct of the airpollution control methods described herein, including the specificpermutations recited above.

In one embodiment, the residue is provided as a solid, substantiallywater insoluble, mercury-containing material, comprising an inorganicmatrix containing mercury in a substantially nonleachable form, producedas a byproduct of an air pollution control process in which amercury-containing combustion gas is allowed to contact a mixture of analkaline-earth metal sulfide and a buffering agent. In contrast, theprocess residues from sodium polysulfide and sodium tetrasulfidescrubbing are not stabilized and are substantively soluble, in that theycontain quantities of sulfates, sulfites and thiosulfates, therebypresenting a waste disposal problem.

The invention has been described in terms of various exemplary andpreferred embodiments, but is not limited thereto. Various modificationscan be made without departing from the invention, the scope of which islimited only by the appended claims and their equivalents. Throughoutthe claims, use of “an” and other singular articles is not intended toproscribe the use of plural components. Thus, more than onealkaline-earth metal sulfide, more than one alkaline-earth metal-basedpH buffer, and so forth, may be utilized.

1. A composition for removing a heavy metal from a combustion gas,comprising: an aerosolized, aqueous dispersion or an aerosolized, slurryof an alkaline-earth metal sulfide and a buffering agent.
 2. Acomposition as recited in claim 1, wherein the aerosolized dispersion oraerosolized slurry comprises a 20-50% w/w solids dispersion ofalkaline-earth metal sulfide and a buffering agent.
 3. A composition asrecited in claim 1, wherein the aerosolized dispersion or aerosolizedslurry is formed using a spray nozzle or rotary atomizer.